The present article is an attempt to bring together the development of mental activity and consciousness in the framework of the organism-environment theory (Jarvilehto, 1998a, 1998b, 1999); the main question is how the development of mental activity and consciousness can be formulated if the starting point is not the separation of man and environment as in traditional cognitive psychology, but a unitary organism-environment system. According to the present formulation, mental activity is conceived as activity of the whole organism-environment system and (...) connected to the general development of life as a specific form of an organism-environment system comprising neurons. The advent of consciousness is regarded as a result of co-operation of such organism-environment systems. Consciousness is based on co-operation for the achievement of common results, and shared by the co-operating individuals (general consciousness), although each individual also makes it concrete from the perspective of his/her own body in the act of participation in common results (personal consciousness). Language is the means of formation of the co-operative system in the achievement of common results, and it is suggested that the use of language is related more to the type of co-operative system and intended common results than to any symbolic representation of the world. It is claimed that on this basis it is possible to develop psychology which takes seriously the concepts of mental activity and consciousness in the description of human action, but does not reduce these concepts either to biological or social factors. The present formulation should be regarded more as a conceptual outline than as a full-blown theory. (shrink)